The present invention relates to high voltage transistors and, more particularly, to radio frequency (xe2x80x9cRFxe2x80x9d) power amplifiers.
High power radio frequency (xe2x80x9cRFxe2x80x9d) power amplifiers are typically employed in semiconductor plasma processing applications. They can also be used in other RF applications, such as radio communications and induction and dielectric heating.
In a typical RF amplifier arrangement, a high power RF source produces an RF wave at a preset frequency which is furnished along a power conduit to a plasma chamber. The RF power is typically provided at a fixed impedance, typically 50 ohms. An RF drive signal is generated and fed to a power amplifier, which provides the RF wave at a desired power level, e.g., 1.25 kW, 2.5 kW, 5 kW, 10 kW, etc. Depending on the application, the RF wave can be provided at different preset frequencies, e.g., 13.56 MHz, 2.0 MHz, 4.0 MHz, 27.12 MHz, or 40.68 MHz. The bandwidth around these preset frequencies is plus or minus 5% to 10% for frequency agile plasma applications.
Conventional plasma generator RF power amplifiers use bipolar or field effect transistors with operating voltages between 40 and 50 volts. These semiconductor devices have semiconductor breakdown voltages in the range of 100 to 150 volts. For example, a 2.5 kW amplifier typically employs eight push-pull pairs of transistors in corporate parallel, with one transistor for each of the forward phase and reverse phase sections of each push-pull pair.
The output of the amplifier is typically classified into a commonly recognized class depending on the conduction characteristics of the amplifier""s active devices. Classification is usually based on the amount of time the active devices operate during one cycle of input voltage. One of the most common classes of amplifier is the class C amplifier. Class C amplifiers are non-linear amplifiers capable of approximately 65% collector or drain efficiency.
Class E amplifiers are another class of amplifiers with switched mode operation capabilities, requiring no RF feedback. High efficiency amplification is achieved by a switching amplifier which, ideally, dissipates no power in the switch. All of the power is, instead, ideally dissipated in the load. An ideal class E amplifier dissipates no power because there is no voltage across the switching device when it is on and no current flowing through when it is off. By operating the transistor as a switch, the instantaneous power loss in the switch is almost zero.
Therefore, the switched amplifier classes, such as class E amplifiers, exhibit highest efficiency. For RF purposes, the switched type amplifiers exhibit high DC to RF efficiency and are suitable for many frequency agile applications. However, switched type amplifiers oftentimes exhibit instability due to the lack of a broadband RF feedback in the power amplifier configuration when operated into high VSWR loads.
Sophisticated cooling techniques are being developed to reduce high junction temperatures in high power RF generators. These sophisticated cooling techniques can include techniques such as heat spreaders or diamond inserts between the silicon die and the heatsink. These approaches aid in removing the heat at the die junctions at significant cost but do not address the cause of the high dissipation itself.
A further approach to alleviate the problem of high die junction temperatures is to use a high efficiency mode of operation with the amplifier such as class E or class F. However, the devices which are used as switches within the RF amplifier (such as bipolar transistors and MOSFETs) require an electrical insulator between the collector (for a bipolar transistor) or drain (for a MOSFET transistor) and the heatsink for improving the die junction temperatures. An added disadvantage is that the heat generated by the transistor is not easily removed. In addition, traditional class E amplifier efficiency degrades rapidly as the load impedance or operating frequency is varied. Therefore, the amplifier""s operating range is limited or large amounts of silicon real estate is necessary to achieve satisfactory operation in environments such as plasma processing.
Therefore, there is a need for a more cost advantageous and reliable implementation of an RF amplifier which operates satisfactorily at very high power density and at high load mismatches. Additionally, there is a need for a switched amplifier which provides for stable performance without compromising output power efficiency.
In accordance with the present invention, a high efficiency RF amplifier with low die temperature for driving high load mismatches and high power density is disclosed. In one aspect, the present invention concerns a radio frequency (xe2x80x9cRFxe2x80x9d) power amplifier with high output efficiency operating in a switched mode at a predetermined frequency band, said amplifier comprising:
a semiconductor device having a control terminal and two conducting terminals, said semiconductor device capable of a conductive state and a nonconductive state, wherein said control terminal controls the conductance across said two conducting terminals, wherein a first of said two conducting terminals is tied to ground potential, wherein a second of said two conducting terminals comprises the output of said amplifier;
a RF source coupled to said control terminal of said semiconductor device;
a resonant inductor circuit coupled to said second of said two conducting terminals, said resonant inductor circuit for eliminating the capacitance between said two conducting terminals when said semiconductor device is in said nonconductive state; and
a filter coupled to said second of said two conducting terminals for providing controlled impedance to signals outside of said predetermined frequency band.
In another aspect, this invention concerns a radio frequency (xe2x80x9cRFxe2x80x9d) power amplifier with high output efficiency operating in a switched mode at a predetermined frequency band, said amplifier comprising:
a discrete transistor having a gate terminal, a source terminal, and a drain terminal, said drain terminal in a grounded configuration, said source terminal comprising the output of said amplifier;
a RF source coupled to said gate terminal of said discrete transistor;
a resonant inductor circuit coupled to said source terminal for eliminating the capacitance between said drain terminal and said source terminal when said discrete transistor is in an off state; and
a filter coupled to said source terminal for filtering out signals outside of said predetermined frequency band.
In another aspect, this invention concerns a radio frequency (xe2x80x9cRFxe2x80x9d) power amplifier with high output efficiency operating in a switched mode at a predetermined frequency band, said amplifier comprising:
a discrete transistor having a gate terminal, a source terminal, and a drain terminal, said source terminal in a grounded configuration, said drain terminal comprising the output of said amplifier;
a RF source coupled to said gate terminal of said discrete transistor;
a resonant inductor circuit coupled to said drain terminal for eliminating the capacitance between said drain terminal and said source terminal when said discrete transistor is in an off state; and
a filter coupled to said drain terminal for filtering out signals outside of said predetermined frequency band.